Field of the Invention
The present invention relates an optical low-pass filter and an image pickup apparatus including the same.
Description of the Related Art
An image pickup apparatus such as a digital still camera and a video camera using a two-dimensional solid image pickup element (image sensor) such as a CCD and a C-MOS sensor uses an optical low-pass filter to prevent color moire and luminance moire of an image. Using the optical low-pass filter can control luminance distribution of a high frequency reaching the image sensor. Four-point separation optical low-pass filters consisted of a horizontal separation birefringent plate and a vertical separation birefringent plate have been most used commonly. The optical low-pass filter of Japanese Patent Laid-open No. 1-254912 and Japanese Patent No. 3204471 utilizes diffraction of an incident light generated by a micro refractive index distribution shape formed on a surface and controls a pitch of the micro refractive index distribution shape to obtain a desired point image distribution width.
However, when a light having a large F-number obliquely enters the optical low-pass filter of Japanese Patent Laid-open No. 1-254912 and Japanese Patent No. 3204471, there are fears that negative effects such as shadows and misregistrations due to edges of the refractive index distribution shape occur. This is because a diameter of the light having the large F-number is equivalent to a structure of the optical low-pass filter, and diffraction of the incident light generated by the refractive index distribution shape fails to occur.
Additionally, each of the commonly used four-point separation optical low-pass filters has a MTF shape different in horizontal and diagonal directions, and a MTF in the horizontal direction falls into 0 at a space frequency (trap point) corresponding to a separation width of four-point separation of 0.5 times. Then, if color moire in the diagonal direction is controlled, the separation width cannot be sufficiently widened and resolution in the horizontal direction falls compared with resolution in the diagonal direction.
Further, the four-point separation optical low-pass filters and the optical low-pass filter of Japanese Patent Laid-open No. 1-254912 and Japanese Patent No. 3204471 cannot prevent luminance moire of a low frequency generated at a space frequency that is approximately twice of the Nyquist frequency.